The present invention relates actuators and apparatus the operation of which are based on a method for controlling the twin orientation of the actuator material by the magnetic field. The aim is to produce shape changes, motion and force using those actuators.
Control of motion (and force) is one of the basic elements in mechanical engineering. Development of new materials has made it possible to produce motion and force using special functional materials called actuator materials. The most important groups of actuator materials available are piezoelectric ceramics, magnetostrictive intermetallics, and shape memory alloys. Piezoelectric ceramics develop strains when subjected to an electric field. Frequency response of these materials is fast, but the strain amplitudes are very small, which limits their applicability. Magnetostrictive materials are strained when a magnetic field is imposed on them. Certain high-magnetostrictive intermetallics (e.g., Terfenol-D, Etrema Products, Inc., Ames, Iowa, USA) offer strains up to 0.17%, which is an order of magnitude higher than those of the current piezolectrics. The frequency response of the magnetostrictive intermetallics is lower than that of the piezoelectrics.
Shape memory metals are materials which, when plastically deformed at one temperature, can recover their original undeformed state upon raising their temperature above an alloy-specific transformation temperature. In these materials, crystal structure undergoes a phase transformation into, and out of, a martensite phase when subjected to mechanical loads or temperature. The process when a mechanically deformed shape memory material returns to its original form after heating is called a one-way shape memory effect. Cooling the material subsequently will not reserve the shape change. The one-way shape memory effect is utilized in fastening, tightening and pre-stressing devices. Strains of several percent can be completely recovered, and recovery stresses of over 900 MPa have been attained. In the case of the two-way effect, no deformation is required, and the material xe2x80x9cremembersxe2x80x9d two configurations that are obtained by heating and cooling to alloy-specific temperatures. The temperature difference between the two configurations can be as small as 1 to 2 K. Materials that exhibit a two-way shape memory effect are used to develop forces and displacements in actuators. Those actuators are applied in machinery, robotics and biomedical engineering. The most extensively used shape memory materials are Ni-Ti and Cu-based alloys. A drawback of the shape memory actuators is their slow response due to the thermal control (especially in cooling) and low efficiency (energy conversion), which in many alloys is only about one percent.
In order the shape memory effect to occur, the material must exhibit a twinned substructure. The shape change of the shape memory material is based on the reorientation of the twin structure in the external stress field. A two-dimensional illustration of the twin reorientation is presented in FIG. 1. FIG. 1(a) shows two twin variants, denoted by 1 and 2, with equal proportions in the absence of the external stress. When the stress is applied (FIG. 1(b)), the twin boundaries move and variant 2 grows at the expense of variant 1, producing the shape which better accommodates the applied stress. The result of moving a twin boundary is thus to convert one twin variant into another. That variant will be grown which is most favorably oriented to the applied stress. Ultimately, a single variant of martensite can be produced by straining a sufficient amount, as illustrated in FIG. 1(c). In the martensite phase, the variants are usually oriented in several crystallographic directions. Therefore, complex shape changes of the material can be produced by the reorientation of the twin structure, and a full shape recovery will be obtained. Crystallographic analysis has shown that the boundaries between the martensite plates also behave as twin boundaries, i.e., the individual plates of martensite themselves are twins with respect to adjoining plates. Thus the term twin boundaries, generally refers to the boundaries between martensite plates as well as the boundaries between the boundaries within the plates (this definition also concerns the magnetically controlled twin boundaries discussed below). In some materials, applied stress induces formation of the martensite phase whose twinned substructure is preferentially oriented according to the applied stress. It was recently calculated that magnetic field that is oriented parallel with twin boundary direction produces targestmagnetic-field induced strains in a material.
Reorientation of the twin structure is responsible for the recoverable strains of several percent in appropriate materials (e.g., close to 10 percent in Ni-Ti shape memory alloys). In some alloys the stress required to reorient the twin structure is very low. FIG. 2 shows the stress-strain curves for the selected shape memory materials. It is seen that strains of 4 percent are attained by stresses of 20 to 50 MPa in most of those alloys. Stresses as low as 1 to 30 MPa result in the strains of one percent.
Strain energy densities needed to produce the strain of 1 percent by the reorientation of the twin variants are the areas restricted by the stress-strain curves, strain axis and the vertical dashed line in FIG. 2. The strain energy densities for the alloys In-TI, Nixe2x80x94Mnxe2x80x94Ga (ferromagnetic Ni2MnGa), Cuxe2x80x94Znxe2x80x94Ga and Cuxe2x80x94Zn are 104, 8.5xc3x97104, 1.1xc3x97105 and 2.3xc3x97105 J/m3, respectively.
In the following, magnetic anisotropy energy is introduced, because it plays an important role in the present invention. In ferromagnetic crystais magnetocrystalline anisotropy energy is an energy which directs the magnetization along certain definite crystallographic axes called directions of easy magnetization. FIG. 3a shows the magnetization curves of single crystalline cobalt which has a hexagonal crystal structure. Its easy direction of magnetization is the parallel axis of the unit cell. The saturation is reached at a low magnetic field value in this direction, as shown in FIG. 3a. Saturating the basal plane direction is called a hard direction of magnetization. Magnetic anisotropy energy density corresponding to the magnetization processes in different directions is the area between the magnetization curves for those directions. In cobalt the energy density needed to saturate the sample in the hard direction is about 5xc3x97105 J/m3 (the area between the saturation curves in FIG. 3a). Anisotropy energy densities of magnetically hard Fe- and Co-based alloys range from 105 up to 107 J/m3. The highest anisotropy energy densities (K1 values), close to 108 j/m3, are in 4f metals at low temperatures. In intermetallic compounds such as Co5Nd, Fe14Nd2B and Sm2Co17 the anisotropy energy densities at room temperature are as high as 1.5xc3x97107, 5xc3x97107 and 3.2xc3x97106 J/m3, respectively.
Reorientation of the twin structure by the applied magnetic field. In crystalline ferromagnetic materials, magnetization vectors lie along directions of easy magnetization in the absence of the external magnetic field. This situation is shown in FIG. 4(a) for two twin variants. The easy direction of magnetization is parallel with the side of the unit cells of each variant. It is emphasized that the easy direction does not necessarily be parallel with the side of the unit cell but it can also be any other crystallographic direction characteristic of the material.
When an external magnetic field is applied on a crystalline ferromagnetic material, the magnetization vectors tend to turn from the easy direction of the unit cell to the direction of the external magnetic field. If the magnetocrystalline anisotropy energy, denoted by Uk in this presentation, is high, the magnetic field strengths required to turn the magnetization off from the easy directions are also high, as was illustrated for hexagonal cobalt in FIG. 3. If the energy of turning the twin variants, (i.e., the energy of the motion of the twin boundaries) is low enough compared to the magnetocrystalline anisotropy energy Uk, the twin variants are turned by the external magnetic field, and the magnetization remains in the original easy direction of the unit cells. FIG. 4b shows how the unit cells of one variant are turned into another by the external magnetic field (magnetization is assumed to turn parallel with the external field direction in this presentation for illustration). As a result, twins in favourable orientation to the magnetic field grow at the expense of the other twins, as shown in FIG. 5. FIG. 5(a) represents the starting situation in the absence of the field when the twin variants with equal proportions are present. FIG. 5(b) shows how the unit cells whose easy direction of magnetization is off from the direction of the external magnetic field are turned along with the field direction. This results in the growth of the favourably oriented twin variant and the decrease of the other variant. Ultimately, only one twin variant may remain, as shown in FIG. 5(c).
The magnetic-field-control of the reorientation of the twin variants is expected to produce recoverable strains of several percent in appropriate materials (in way analogous to stress-induced recoverable strains in the shape memory alloys). To reach a certain magnetically induced strain, it is necessary that the magnetocrystalline anisotropy energy Uk of the material is larger than the energy needed to reorient the twin variants to achieve this strain. The latter energy, defined as the energy of the reorientation of the twin structure, and denoted by Etw, includes also strain and dissipation energy terms related to the shape change of the material. The velocity of the twin boundaries is very fast in many materials, even a fraction of the speed of sound. This means that the magnetic-field-induced strokes are very fast in such materials. The magnetic-field induced strains were demonstrated in nonstoichiometric Ni2MnGa alloys.
This invention concerns certain magnetically driven actuators and apparatus that produce motion and force and also such apparatus that monitor shape changes of an active element. The operation of the actuator is based on the magnetic-field-controlled reorientation of the twin structure of the material of the actuating element of the actuator that generates the motion. This kind of actuators can produce strains of several percent (as large as the shape memory materials). Because of the magnetic control of the these actuators, the response times are much faster, control more precise, and efficiency better than those of the shape memory materials. The new magnetically driven actuators will exhibit a great potential in mechanical engineering. They will replace hydraulic, pneumatic and electromagnetic drives in many applications. Employment of these actuators leads to simpler, lighter, and more reliable constructions than use of conventional technology. Because the twin reorientation occurs in three dimensions, complex shape changes can be produced under the magnetic control. Applicability of this invention is expanded by the possibility for controlling and supplying the power of the actuators at a distance. The whole machine developing a controlled motion or desired shape changes (e.g., bending, shear, twisting, clipping, fastening, pumping) may be a small appropriately shaped and preoriented piece of material. Due to the small twin size in many materials, this invention is expected to have great potential also in micro- and nanotechnology. Linear and rotary motors, pumps, valves, couplers, vibrators and many other equipment are also concerned by this invention. In an inverse effect, magnetic field is changed when the active element made from the material according to the present invention is deformed. This can be used to monitor the state of the shape of the active element, e.g, in positioning devices, joysticks, keyboards, stress sensors and electric power generators.